High flux constant voltage LED modules can be driven with 2-wire Pulse Width Modulation (PWM) dimmers by resorting to either of two basic arrangements.
In a first arrangement, a PWM input voltage is imposed directly on the power line. The LED or LEDs are turned on only when the voltage is higher than a certain threshold, otherwise they are off. Resorting to this arrangement makes it impossible to supply any ancillary circuitry (such as logic circuitry, e.g. a microcontroller for power-line communications) provided in the module if the dimming level is very low. Additionally, state-of-the art high flux LED modules typically include a step-up or step-down converter. These converters require huge capacitors at the input side for stability and filtering reasons. Consequently, if the module discharges these capacitors during the low input voltage period, when the input voltage goes high, a high current spike is produced on the power line. The possibility of dimming such LED modules with a direct PWM modulation of the input voltage has been so far restricted to limiting the input current, thus decreasing the performance at low dimming levels, or to avoiding the low dimming condition.
In an alternative arrangement, the power line and the PWM signal are separated and are carried by different wires. In that case, the PWM signal is transmitted on a third wire, while a controller in the module provides the PWM output current to the LEDs accordingly to such signal. In this way, ancillary control circuitry in the module can be supplied also at low dimming levels, at the cost of a more complex wiring.